1. Technical Field
Embodiments described herein relate to a device for calculating impedances of a battery cell for respective frequency domains and a battery impedance measuring system.
2. Related Art
Secondary batteries that are charged repetitively have been widely used as a driving power source for a drive motor of a hybrid vehicle, an electric vehicle, or the like. Furthermore, secondary batteries have been also widely used in industrial fields, public institutions, general households, or the like from the viewpoint that the secondary battery may store energy which is obtained from solar power generation, wind power generation, or the like with a relatively small environmental load, without depending on fossil fuel.
In general, these secondary batteries are configured as a battery module in which a predetermined number of battery cells from which a desired output voltage is obtained are connected in series, or as a battery pack in which a predetermined number of battery modules from which a desired output voltage is obtained are connected in parallel to obtain a desired current capacity (AH).
However, from a convenience aspect of a charging time, traveling distance, and the like, currently, it is believed that the lithium ion battery will become the mainstream for secondary batteries that are mounted in vehicles as a driving power source for a driving motor.
FIG. 10 shows a block diagram illustrating an example of a battery system using the secondary battery in the related art. In FIG. 10, a battery module 10 is configured in such a manner that a plurality of battery cells 111 to 11n and a current sensor 12 are connected in series, and the battery module 10 is connected to a load L in series.
A battery monitoring device 20 includes a plurality of A/D converters 211 to 21n+1 that are provided to independently correspond to the plurality of battery cells 111 to 11n and the current sensor 12 that make up the battery module 10, and a processing device 23 to which output data of the A/D converters 211 to 21n+1 is input via an internal bus 22.
An output voltage of each of the battery cells 111 to 11n of the battery module 10 and a detection signal of the current sensor 12 of the battery module 10 are input to the corresponding A/D converters 211 to 21n+1 and are converted to digital signals, and the output data of the A/D converters 211 to 21n+1 is input to the processing device 23 via the internal bus 22.
The processing device 23 obtains, for example, an internal resistance value of each of the battery cells 111 to 11 based on the output data of the A/D converters 211 to 21n+1, estimates a value corresponding to voltage drop at the time of taking out a desired current from the internal resistance value, and transmits this data to a host battery system controller 40 via an external bus 30.
The battery system controller 40 controls the battery module 10 and the load device L in order to stably drive the load device L by a current output voltage of the battery module 10 based on the data input from the battery monitoring device 20.
As an index of evaluating performance of the secondary battery making up the battery module 10, an internal impedance characteristic shown in FIGS. 11 and 12 may be exemplified. FIG. 11 shows a diagram illustrating an impedance characteristic example in a case where a fully charged battery is left as is in a high-temperature state, and FIG. 12 shows a diagram illustrating an impedance characteristic example in the case of repeated charge and discharge in a high-temperature state. In addition, in FIGS. 11 and 12, the left-side drawing illustrates a Cole-Cole plot in which complex impedance based on an AC impedance measurement result is plotted in complex coordinates, and the right-side drawing illustrates a Bode diagram showing an impedance frequency characteristic.
The left-side drawing of FIG. 11 shows a process in which the period left increases, for example, for one year, for two years, . . . , AC impedance increases. The left-side drawing of FIG. 12 shows a process in which as the charge and discharge is repeated, for example, for 50 times, for 100 times, . . . , the AC impedance increases.
As the impedance increases, voltage drop of a battery increases when producing a current, and thus a sufficient output voltage may not be obtained. A low-frequency portion of the right-side drawing corresponds to a case in which an accelerator of a vehicle is continuously pressed for a long time. From this data, since the impedance increases at the low-frequency portion, it may be assumed that the voltage drop gradually increases. That is, an output characteristic varies according to deterioration of the battery, and thus a sufficient output may not be produced.
FIG. 13 shows a block diagram illustrating an example of a measuring circuit that measures the AC impedance of the secondary battery in the related art, and in FIG. 13, the same reference numerals are attached to the same parts as FIG. 10. In FIG. 13, a sweep signal generator 50 is connected to both ends of a serial circuit of the battery 10 and the current sensor 12. This sweep signal generator 50 outputs an AC signal in which an output frequency varies in a sweeping manner within a range including a frequency characteristic region shown in the right-side drawing of FIGS. 11 and 12 to the serial circuit of the battery 10 and the current sensor 12.
An AC voltage monitor 60 measures AC voltage of both ends of the battery 10, and inputs this AC voltage to an impedance calculator 80. An AC current monitor 70 measures an AC current that flows to the current sensor 12 and inputs this AC current to the impedance calculator 80.
The impedance calculator 80 calculates complex impedance of the battery 10, which is a ratio between a measured voltage of the AC voltage monitor 60 at each frequency of the output signal of the sweep signal generator 50 and a measured current of the AC current monitor 70. The calculated complex impedance is plotted on the complex plane, thereby obtaining the Cole-Cole plot shown in FIGS. 11 and 12.
From the Cole-Cole plot that is created in this way, for example, each parameter of an equivalent circuit of the battery 10 as shown in FIG. 14 may be estimated. In addition, in the equivalent circuit of FIG. 14, a DC power source E, a resistor R1, a parallel circuit of a resistor R2 and a capacitor C2, a parallel circuit of a resistor R3 and a capacitor C3, and a parallel circuit of a resistor R4 and an inductance L4 are connected in series. JP-A-2003-4780 discloses in detail measurement of impedance by an alternating current method together with an automatic measurement method.
As described above, since various kinds of information of a battery may be obtained through measurement of the internal impedance characteristic of the battery, when the internal impedance characteristic of the battery may be measured on the spot, such as a vehicle, a power plant, a household power storage system, and the like that actually use the battery, a present state of the battery may be ascertained based on the information, and the battery may be controlled so as to be effectively used to the maximum depending on the present state of the battery.
However, in the system configuration in the related art shown in FIG. 10, the internal resistance value of each of the battery cells 111 to 11n may be obtained, but since data communication between the processing device 23 and the battery system controller 40 is performed intermittently, voltage data of each of the battery cells 111 to 11n becomes discrete data of which a period is, for example, 100 ms or more.
As a result thereof, only a state of each of the battery cells 111 to 11n may be detected with reference to a table including a voltage, a current, a temperature, and the like, and the internal impedance characteristic of each of the battery cells 111 to 11n in which many pieces of information are collectively contained may not be measured.
In addition, according to the measuring circuit in the related art as shown in FIG. 13, the sweep signal generator 50 is necessary, and it is difficult to mount the measuring circuit as shown in FIG. 13 in each of the on-site cells in terms of both cost and space.